Bit shifting for cross-component adaptive loop filtering for video coding

ABSTRACT

An example method includes decoding a plurality of filter coefficients of a cross-component adaptive loop filter, wherein decoding a particular filter coefficient of the plurality of filter coefficients comprises: decoding, from an encoded video bitstream, a syntax element specifying an exponent value that represents a log base 2 of an absolute value of the particular filter coefficient as two raised to the power of the exponent value; and determining a value of the particular filter coefficient based on the exponent value; reconstructing samples of a block of video data; and cross-component adaptive loop filtering, based on the plurality of filter coefficients, the block of video data.

This application claims the benefit of U.S. Provisional Application No.62/904,508, filed Sep. 23, 2019, the entire contents of which areincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video coding, including video encoding andvideo decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In general, this disclosure describes techniques related tocross-component adaptive loop filtering (CC-ALF) of video data. Toperform ALF, a video coder may separately filter corresponding luma andchroma blocks using different coefficient sets (e.g., using a lumacoefficient set to filter the luma block and one or more chromacoefficient sets to filter the chroma blocks). However, a luma block mayinclude details that may be lost in a corresponding chroma block in acoding loop. As such, the video coder may perform CC-ALF in-whichinformation from a luma block is used to enhance a corresponding chromablock.

For example, a video coder may filter the luma block with a first set ofchroma filter coefficients to generate an intermediate block for a firstchroma component (e.g., Cb) and filter the luma block with a second setof chroma filter coefficients to generate an intermediate block for asecond chroma component (e.g., Cr). A video encoder may signal values ofthe filter coefficients use for CC-ALF (e.g., at least the first andsecond sets of chroma filter coefficients used to filter the luma block)to a video decoder as one or more syntax elements in an encoded videobitstream. The video coder may then add the respective intermediateblocks to ALF filtered chroma blocks of the chroma components.

To filter the luma block to generate an intermediate block for a chromacomponent, the video coder may perform a plurality of multiplicationoperations for each sample of the luma block. For instance, the videocoder may calculate a filtered value for a particular sample of the lumablock as a summation of chroma filter coefficients multiplied by samplesof the luma block. As such, performing CC-ALF may involve a large numberof multiplication operations (e.g., seven for an 8×8 luma block).Performance of such a high number of multiplication operations may be aresource intensive endeavor for the video coder, which may undesirablyincrease coding time and/or power consumption.

In accordance with one or more techniques of this disclosure, a videocoder may code (e.g., a video encoder may encode and a video decoder maydecode) filter coefficients for CC-ALF such that absolute values of thefilter coefficients are restricted to be zero or a power of two. Whenfiltering the luma block using the filter coefficients to generate theintermediate chroma blocks, the video coder may replace themultiplication operations with bit-shift operations (e.g., left-shiftand right-shift operations). Because the absolute values of the filtercoefficients are restricted to be zero or a power of two, replacement ofthe multiplication operations with bit-shift operations may bemathematically equivalent (i.e., yield an identical intermediate chromablock). However, while mathematically equivalent, the bit-shiftoperations may be substantially less resource-intensive than themultiplication operations. Additionally, when implemented in dedicatedhardware (e.g., an application specific integrated circuit (ASIC)), thehardware needed to perform bit-shift operations may be simpler than thehardware needed to perform multiplication operations. In this way, thetechniques of this disclosure reduce the resource requirements ofCC-ALF.

As one example, a method includes decoding a plurality of filtercoefficients of a cross-component adaptive loop filter, wherein decodinga particular filter coefficient of the plurality of filter coefficientscomprises: decoding, from an encoded video bitstream, a syntax elementspecifying an exponent value that represents a log base 2 of an absolutevalue of the particular filter coefficient as two raised to the power ofthe exponent value; and determining a value of the particular filtercoefficient based on the exponent value; reconstructing samples of ablock of video data; and cross-component adaptive loop filtering, basedon the plurality of filter coefficients, the block of video data.

As another example, a method includes encoding values of a plurality offilter coefficients of a cross-component adaptive loop filter, whereinencoding a value of a particular filter coefficient of the plurality offilter coefficients comprises: encoding, in an encoded video bitstream,a syntax element specifying an exponent value that represents a log base2 of an absolute value of the particular filter coefficient as tworaised to the power of the exponent value; reconstructing samples of ablock of video data; and cross-component adaptive loop filtering, basedon the values of the plurality of filter coefficients, the block ofvideo data.

As another example, a device includes a memory configured to store atleast a portion of an encoded video bitstream; and one or moreprocessors that are implemented in circuitry and configured to: decode aplurality of filter coefficients of a cross-component adaptive loopfilter, wherein, to decode a particular filter coefficient of theplurality of filter coefficients, the one or more processors areconfigured to: decode, from the encoded video bitstream, a syntaxelement specifying an exponent value that represents a log base 2 of anabsolute value of the particular filter coefficient as two raised to thepower of the exponent value; and determine a value of the particularfilter coefficient based on the exponent value; reconstruct samples of ablock of video data; and cross-component adaptive loop filter, based onthe plurality of filter coefficients, the block of video data.

As another example, a device includes a memory configured to store atleast a portion of an encoded video bitstream; and one or moreprocessors that are implemented in circuitry and configured to: encodevalues of a plurality of filter coefficients of a cross-componentadaptive loop filter, wherein, to encode a value of a particular filtercoefficient of the plurality of filter coefficients, the one or moreprocessors are configured to: encode, in the encoded video bitstream, asyntax element specifying an exponent value that represents a log base 2of an absolute value of the particular filter coefficient as two raisedto the power of the exponent value; reconstruct samples of a block ofvideo data; and cross-component adaptive loop filter, based on thevalues of the plurality of filter coefficients, the block of video data.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a block diagram illustrating an example filter unit, inaccordance with one or more techniques of this disclosure.

FIG. 6 is a flowchart illustrating an example method for encoding acurrent block in accordance with the techniques of this disclosure.

FIG. 7 is a flowchart illustrating an example method for decoding acurrent block in accordance with the techniques of this disclosure.

FIG. 8 is a flowchart illustrating an example method for cross-componentadaptive loop filtering (CC-ALF) on a current block in accordance withone or more techniques of this disclosure.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,uncoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch as smartphones, televisions, cameras, display devices, digitalmedia players, video gaming consoles, video streaming device, or thelike. In some cases, source device 102 and destination device 116 may beequipped for wireless communication, and thus may be referred to aswireless communication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for performingcross-component adaptive loop filtering. Thus, source device 102represents an example of a video encoding device, while destinationdevice 116 represents an example of a video decoding device. In otherexamples, a source device and a destination device may include othercomponents or arrangements. For example, source device 102 may receivevideo data from an external video source, such as an external camera.Likewise, destination device 116 may interface with an external displaydevice, rather than including an integrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques forcross-component adaptive loop filtering. Source device 102 anddestination device 116 are merely examples of such coding devices inwhich source device 102 generates coded video data for transmission todestination device 116. This disclosure refers to a “coding” device as adevice that performs coding (encoding and/or decoding) of data. Thus,video encoder 200 and video decoder 300 represent examples of codingdevices, in particular, a video encoder and a video decoder,respectively. In some examples, source device 102 and destination device116 may operate in a substantially symmetrical manner such that each ofsource device 102 and destination device 116 include video encoding anddecoding components. Hence, system 100 may support one-way or two-wayvideo transmission between source device 102 and destination device 116,e.g., for video streaming, video playback, video broadcasting, or videotelephony.

In general, video source 104 represents a source of video data (i.e.,raw, uncoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although shown separately from video encoder 200 and videodecoder 300 in this example, it should be understood that video encoder200 and video decoder 300 may also include internal memories forfunctionally similar or equivalent purposes. Furthermore, memories 106,120 may store encoded video data, e.g., output from video encoder 200and input to video decoder 300. In some examples, portions of memories106, 120 may be allocated as one or more video buffers, e.g., to storeraw, decoded, and/or encoded video data.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. File server 114and input interface 122 may be configured to operate according to astreaming transmission protocol, a download transmission protocol, or acombination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., storage device 112,file server 114, or the like). The encoded video bitstream may includesignaling information defined by video encoder 200, which is also usedby video decoder 300, such as syntax elements having values thatdescribe characteristics and/or processing of video blocks or othercoded units (e.g., slices, pictures, groups of pictures, sequences, orthe like). Display device 118 displays decoded pictures of the decodedvideo data to a user. Display device 118 may represent any of a varietyof display devices such as a cathode ray tube (CRT), a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 6),” Joint Video Experts Team (WET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting:Gothenburg, SE, 3-12 Jul. 2019, JVET-02001-vE (hereinafter “VVC Draft6”). The techniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to JEM or VVC. According to JEM or VVC,a video coder (such as video encoder 200) partitions a picture into aplurality of coding tree units (CTUs). Video encoder 200 may partition aCTU according to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) partitions. A triple tree partition is apartition where a block is split into three sub-blocks. In someexamples, a triple tree partition divides a block into three sub-blockswithout dividing the original block through the center. The partitioningtypes in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of JEM and VVC also provide an affine motion compensationmode, which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofJEM and VVC provide sixty-seven intra-prediction modes, includingvarious directional modes, as well as planar mode and DC mode. Ingeneral, video encoder 200 selects an intra-prediction mode thatdescribes neighboring samples to a current block (e.g., a block of a CU)from which to predict samples of the current block. Such samples maygenerally be above, above and to the left, or to the left of the currentblock in the same picture as the current block, assuming video encoder200 codes CTUs and CUs in raster scan order (left to right, top tobottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of thecoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) coefficients at the front of the vector and to place lowerenergy (and therefore higher frequency) transform coefficients at theback of the vector. In some examples, video encoder 200 may utilize apredefined scan order to scan the quantized transform coefficients toproduce a serialized vector, and then entropy encode the quantizedtransform coefficients of the vector. In other examples, video encoder200 may perform an adaptive scan. After scanning the quantized transformcoefficients to form the one-dimensional vector, video encoder 200 mayentropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information of apicture into CTUs, and partitioning of each CTU according to acorresponding partition structure, such as a QTBT structure, to defineCUs of the CTU. The syntax elements may further define prediction andresidual information for blocks (e.g., CUs) of video data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

As discussed above and in accordance with one or more techniques of thisdisclosure, video encoder 200 and/or video decoder 300 may be configuredto signal filter coefficients for CC-ALF with absolute values restrictedto being zero or powers of two. In this way, video encoder 200 and/orvideo decoder 300 may replace multiplication operations in theperformance of CC-ALF with bit-shift operations, which are less resourceintensive.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), they can be furtherpartitioned by respective binary trees. The binary tree splitting of onenode can be iterated until the nodes resulting from the split reach theminimum allowed binary tree leaf node size (MinBTSize) or the maximumallowed binary tree depth (MaxBTDepth). The example of QTBT structure130 represents such nodes as having dashed lines for branches. Thebinary tree leaf node is referred to as a coding unit (CU), which isused for prediction (e.g., intra-picture or inter-picture prediction)and transform, without any further partitioning. As discussed above, CUsmay also be referred to as “video blocks” or “blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs, and are further processed according toprediction and transform without further partitioning.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 video coding standard in development. However,the techniques of this disclosure are not limited to these video codingstandards, and are applicable generally to video encoding and decoding.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video encoder 200 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that can beprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, one or more of the units may bedistinct circuit blocks (fixed-function or programmable), and in someexamples, the one or more units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theobject code of the software that video encoder 200 receives andexecutes, or another memory within video encoder 200 (not shown) maystore such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,uncoded version of the current block from video data memory 230 and theprediction block from mode selection unit 202. Residual generation unit204 calculates sample-by-sample differences between the current blockand the prediction block. The resulting sample-by-sample differencesdefine a residual block for the current block. In some examples,residual generation unit 204 may also determine differences betweensample values in the residual block to generate a residual block usingresidual differential pulse code modulation (RDPCM). In some examples,residual generation unit 204 may be formed using one or more subtractorcircuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit 202 does not further partition aCU into PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 120 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as intra-block copy mode coding,affine-mode coding, and linear model (LM) mode coding, as some examples,mode selection unit 202, via respective units associated with the codingtechniques, generates a prediction block for the current block beingencoded. In some examples, such as palette mode coding, mode selectionunit 202 may not generate a prediction block, and instead generatessyntax elements that indicate the manner in which to reconstruct theblock based on a selected palette. In such modes, mode selection unit202 may provide these syntax elements to entropy encoding unit 220 to beencoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the transform coefficient blocksassociated with the current block by adjusting the QP value associatedwith the CU. Quantization may introduce loss of information, and thus,quantized transform coefficients may have lower precision than theoriginal transform coefficients produced by transform processing unit206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples. Filterunit 216 may perform cross-component adaptive loop filtering (CC-ALF)techniques of this disclosure, alone or in any combination. Forinstance, filter unit 216 may perform CC-ALF as discussed below withreference to FIG. 5. Filter unit 216 may generate one or morecoefficients for CC-ALF. For instance, filter unit 216 may generate afirst set of filter coefficients to be used when generating a firstintermediate chroma block from a luma block and a second set of filtercoefficients to be used when generating a second intermediate chromablock from the luma block. As discussed above and in accordance with oneor more techniques of this disclosure, filter unit 216 may restrictabsolute values of the generated filter coefficients to be zero or apower of two (e.g., 1, 2, 4, 8, 16, 32, 64, 128, 256, etc.). Likewise,entropy encoding unit 220 may be configured to entropy encodecross-component adaptive loop filtering parameters according to thetechniques of this disclosure. For instance, as opposed to encoding theactual values of the filter coefficients, entropy encoding unit 220 mayencode an exponent value of the filter coefficients and a video decodermay reconstruct the actual values of the filter coefficients based onthe exponent value.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding block andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data and that includes a memory configured to store video data,and one or more processing units implemented in circuitry and configuredto perform cross-component adaptive loop filtering techniques accordingto this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 according tothe techniques of JEM, VVC, and HEVC. However, the techniques of thisdisclosure may be performed by video coding devices that are configuredto other video coding standards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video decoder 300 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeadditional units to perform prediction in accordance with otherprediction modes. As examples, prediction processing unit 304 mayinclude a palette unit, an intra-block copy unit (which may form part ofmotion compensation unit 316), an affine unit, a linear model (LM) unit,or the like. In other examples, video decoder 300 may include more,fewer, or different functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. CPB memory 320and DPB 314 may be provided by the same memory device or separate memorydevices. In various examples, CPB memory 320 may be on-chip with othercomponents of video decoder 300, or off-chip relative to thosecomponents.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to be executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can be programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, the one ormore units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, the one or more units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the transform coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Entropy decoding unit 302 may further entropy decode cross-componentadaptive loop filter parameters according to the techniques of thisdisclosure. For instance, in accordance with one or more techniques ofthis disclosure, entropy decoding unit 302 may decode, for each of aplurality of filter coefficients and from the encoded video bitstream, asyntax element specifying an exponent value that represents a log base 2of an absolute value of the particular filter coefficient as two raisedto the power of the exponent value. Where the exponent value for aparticular filter coefficient is non-zero, entropy decoding unit 302 maydecode, from the encoded video bitstream and for the particular filtercoefficient, a syntax element with a value that specifies a sign (e.g.,either positive or negative) of the particular filter coefficient.Entropy decoding unit 302 may reconstruct values of the plurality offilter coefficients based on the exponent values. For instance, entropydecoding unit 302 may reconstruct a value of the particular filtercoefficient in accordance with the following equation:c(i)=sign(i)*2^(c′(i)+1)

where c(i) is the value of the particular filter coefficient, sign(i) isnegative one where the signaled sign is negative and positive one wherethe signaled sign is positive, and c′(i) is the signaled exponent valuefor the particular filter coefficient.

Entropy decoding unit 302 may provide the reconstructed cross-componentadaptive loop filter coefficients to filter unit 312. Filter unit 312may perform one or more filter operations on reconstructed blocks. Forexample, filter unit 312 may perform deblocking operations to reduceblockiness artifacts along edges of the reconstructed blocks. Operationsof filter unit 312 are not necessarily performed in all examples.According to the techniques of this disclosure, filter unit 312 may usethe cross-component adaptive loop filter coefficients to performcross-component adaptive loop filtering of a decoded block of videodata.

Video decoder 300 may store the reconstructed blocks in DPB 314. Asdiscussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures fromDPB 314 for subsequent presentation on a display device, such as displaydevice 118 of FIG. 1.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured toperform cross-component adaptive loop filtering techniques of thisdisclosure, alone or in any combination.

FIG. 5 is a block diagram illustrating an example filter unit, inaccordance with one or more techniques of this disclosure. Filter unit500 of FIG. 5 may be considered to be an example of filter unit 216 ofvideo encoder 200 or filter unit 312 of video encoder 300.

Filter unit 500 may include components configured to perform varioustypes of filtering. For instance, as shown in FIG. 5, filter unit 500may include components configured to perform sample adaptive offset(SAO) filtering, such as SAO luma filter 502, SAO Cb filter 504, and SAOCr filter 506. As also shown in FIG. 5, filter unit 500 may includecomponents configured to perform cross-component adaptive loop filtering(CC-ALF), such as ALF luma filter 508, CC ALF Cb filter 510, CC ALF Crfilter 512, ALF chroma filter 514, adder 516, and adder 518.

In operation, SAO luma filter 502 may receive an input luma block ofvideo data, perform SAO filtering on the input luma block to generate anoutput luma block of video data, and provide the output luma block ofvideo data to one or more other filter components, such as ALF lumafilter 508, CC ALF Cb filter 510, and CC ALF Cr filter 512. SAO Cbfilter 504 may receive an input Cb chroma block of video data, performSAO filtering on the input Cb chroma block to generate an output Cbchroma block of video data, and provide the output Cb chroma block ofvideo data to one or more other filter components, such as ALF chromafilter 514. Similarly, SAO Cr filter 506 may receive an input Cr chromablock of video data, perform SAO filtering on the input Cr chroma blockto generate an output Cr chroma block of video data, and provide theoutput Cr chroma block of video data to one or more other filtercomponents, such as ALF chroma filter 514.

The ALF components may perform ALF on the blocks of video data providedby the SAO filtering components. For instance, ALF luma filter 508 mayperform adaptive loop filtering on the luma block provided by SAO lumafilter 502 to generate an output luma block, denoted as Y. Additionally,ALF chroma filter 514 may perform adaptive loop filtering on the chromablock provided by SAO Cb filter 504 and SAO Cr filter 506 to generateoutput chroma blocks, denoted as Cb′ and Cr′.

Misra, et al. “Cross-Component Adaptive Loop Filter for chroma” JointVideo Experts Team (WET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG11, 15^(th) Meeting: Gothenburg, SE, 3-12 Jul. 2019, JVET-00636(hereinafter “JVET-00636”) proposed a tool called cross-componentadaptive loop filter (CC-ALF). CC-ALF operates as part of the adaptiveloop filter (ALF) and makes use of luma samples to refine each chromacomponent. For instance, CC ALF Cb filter 510 and CC ALF Cr filter 512may each generate an enhancement/refinement chroma block based on theluma block provided by SAO luma filter 502 (e.g., CC ALF Cb filter 510may generate enhancement chroma block Cb+ and CC ALF Cr filter 512 maygenerate enhancement chroma block Cr+). Each of CC ALF Cb filter 510 andCC ALF Cr filter 512 may generate their respective enhancement chromablock based on a respective set of filter coefficients (e.g., CC ALF Cbfilter 510 may use a first set of filter coefficients and CC ALF Crfilter 512 may use a second set of filter coefficients). For instance,CC ALF Cb filter 510 may generate chroma block Cb+ in accordance withthe following equation:

${\Delta\;{I_{i}\left( {x,y} \right)}} = {\sum\limits_{{({x,y})} \in S_{i}}{{I_{0}\left( {{x_{C} + x_{0}},{y_{C} + y_{0}}} \right)}{c_{i}\left( {x_{0},y_{0}} \right)}}}$where I_(i) is a filtered block, and I_(o) is an unfiltered block,(x_(C), y_(C)) is luma location (x,y), S_(i) is the filter support inluma for color component Cb, and c_(i)(x₀, y₀) is a filter coefficient.

As shown in the above equation, each of CC ALF Cb filter 510 and CC ALFCr filter 512 may perform many multiplication operations. As discussedabove and in accordance with one or more techniques of this disclosure,these multiplication operations may be replaced by bit-shift operations,which are substantially less resource intensive and/or simpler toimplement in hardware than multiplication operations. For instance, toperform filtering using bit-shift operations, each of CC ALF Cb filter510 and CC ALF Cr filter 512 may utilize the following equation:

${\Delta\;{I_{i}\left( {x,y} \right)}} = {\sum\limits_{{({x,y})} \in S_{i}}\left\{ {{{sign}\left( {c_{i}\left( {x_{0},y_{0}} \right)} \right)}*\left\lbrack {{I_{0}\left( {{x_{C} + x_{0}},{y_{C} + y_{0}}} \right)}{\operatorname{<<}c_{i}^{\prime}}\left( {x_{0},y_{0}} \right)} \right\rbrack} \right\}}$

CC-ALF may be controlled by information in the bitstream, and thisinformation includes the aforementioned filter coefficients for eachchroma component (which may be signaled in adaptation parameter set(APS)) and a mask controlling the application of the filter for blocksof samples. In NET-O0636, each of the filter coefficients is representedas a fixed-point decimal number. In particular, a filter coefficientuses the lower 10 bits to represent the decimal part. Each coefficientis signaled with exponential-Golomb (EG) coding, whose order depends onthe coefficient position in the filter template.

As noted above, this disclosure recognizes that the multiplications ofthe CC-ALF tool described in NET-O0636 can be improved and simplified,e.g., according to any or all of the techniques of this disclosure.Accordingly, video encoder 200 and/or video decoder 300 may beconfigured according to any or all of the techniques of this disclosure,e.g., as described below, in any combination.

According to a first technique of this disclosure, a video coder (e.g.,video encoder 200 and/or video decoder 300) may constrain the values ofsome or all of coefficients for cross-component adaptive loop filters510, 512. For instance, the video coder may constrain (e.g., limitpossible selection of) the values of some or all of the coefficients tobe zero or a number of powers of 2 (e.g., such that no multiplicationsare needed for these coefficients). In some examples, instead of havingto perform multiplications, the video coder (i.e., video encoder 200 orvideo decoder 300) may apply bit shifting to the samples. In oneexample, the video coder may constrain absolute values of allcoefficients to be only be 0 or numbers of powers of 2. In anotherexample, the video coder may constrain the absolute values of somecoefficients to be only 0 or numbers of powers of 2. The informationabout which coefficients of a filter are constrained may be the same forall filters without signaling. Alternatively or additionally, theinformation may be the same for all filters of a color component withoutsignaling. Alternatively or additionally, the information may besignaled in the bitstream (e.g., as one or more syntax elements) for asequence, picture, sub-picture, block, or color component.

In some examples where the video coder signals the information in thebitstream (e.g., to signal the values of those constrainedcoefficients), the video coder may only signal the mapped values (whichis the exponent value with the sign of the non-zero coefficients). Theconstrained coefficients c(i) may be mapped to c′(i) as below,

-   -   If c(i) is equal to 0, c′(i) is 0;    -   Otherwise c′(i)=sign(c(i))*(log₂(abs(c(i))+1), where sign(c(i))        is −1 if c(i) is negative, 1 otherwise.

In some examples, the video coder may utilize any combination offixed-order golomb codes, fixed-length code or unary code to signalc′(i).

In some examples, the video coder may signal (or parse) the absolutevalue of c′(i) first by utilizing any combination of fixed-order golombcodes, fixed-length code or unary code. If c′(i) is not 0, the videocoder may subsequently signal (or parse) sign information afterwards(e.g., after signaling the absolute value of c′(i)).

In some examples, the video coder may convert c′(i) to a non-zero valueby c″(i)=c′(i)−c′_(min)(i), and signal the converted value, wherec′_(min)(i) is the min mapped value for ith coefficient. The videodecoder may parse c″(i), which is a non-negative value. Based on c″(i),the video decoder may calculate c′(i)=c″(i)+c′_(min)(i).

According to a second technique of this disclosure, a video coder may beconfigured to constrain the dynamic range of filter coefficients forcross-component adaptive loop filters 510, 512 to reduce a costmultiplier. Let k be the number of bits used to represent the decimalpart of a coefficient. The dynamic range of a filter coefficient c(i)may be constrained in the open interval (−(1<<(k−j)), (1<<(k−j))−1). Thevideo coder may use any combination of fixed-order golomb codes,fixed-length codes, and/or unary codes to signal c(i). The video codermay signal (or parse) the absolute value of c(i) first. If c(i) is notzero, the video coder may subsequently signal (parse) sign informationfor c(i). Additionally or alternatively, the video coder may convertc(i) may to a non-zero value by c′(i)=c(i)−c_(min)(i). The video codermay then signal the converted value, where c_(min)(i) is the min valuefor ith coefficient. The video decoder may parse c′(i), which is anon-negative value. The video decoder may calculate the value of c(i) asc′(i)+c_(min)(i).

FIG. 6 is a flowchart illustrating an example method for encoding acurrent block in accordance with the techniques of this disclosure. Thecurrent block may comprise a current CU. Although described with respectto video encoder 200 (FIGS. 1 and 3), it should be understood that otherdevices may be configured to perform a method similar to that of FIG. 6.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, uncodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(354). Next, video encoder 200 may scan the quantized transformcoefficients of the residual block (356). During the scan, or followingthe scan, video encoder 200 may entropy encode the coefficients (358).For example, video encoder 200 may encode the coefficients using CAVLCor CABAC. Video encoder 200 may then output the entropy encoded data ofthe block (360).

Video encoder 200 may then decode the current block (362). For example,video encoder 200 may inverse quantize and inverse transform thequantized transform coefficients to reproduce the residual block andcombine the reproduced residual block with the prediction block. Videoencoder 200 may then filter the decoded block (364), e.g., usingcross-component adaptive loop filtering techniques according to thisdisclosure. The entropy encoded data of the block may further include,for example, filter indices indicating which cross-component adaptiveloop filters are selected for the block. Video encoder 200 may thenstore the filtered block (366), e.g., for reference when predicting afuture block to be encoded (and decoded).

FIG. 7 is a flowchart illustrating an example method for decoding acurrent block in accordance with the techniques of this disclosure. Thecurrent block may comprise a current CU. Although described with respectto video decoder 300 (FIGS. 1 and 4), it should be understood that otherdevices may be configured to perform a method similar to that of FIG. 7.

Video decoder 300 may receive entropy encoded data for the currentblock, such as entropy encoded prediction information, entropy encodeddata for coefficients of a residual block corresponding to the currentblock, and entropy encoded cross-component adaptive loop filterinformation for the current block (370). Video decoder 300 may entropydecode the entropy encoded data to determine prediction information forthe current block and to reproduce coefficients of the residual block(372). Video decoder 300 may predict the current block (374), e.g.,using an intra- or inter-prediction mode as indicated by the predictioninformation for the current block, to calculate a prediction block forthe current block. Video decoder 300 may then inverse scan thereproduced coefficients (376), to create a block of quantized transformcoefficients. Video decoder 300 may then inverse quantize and inversetransform the coefficients to produce a residual block (378). Videodecoder 300 may ultimately decode the current block by combining theprediction block and the residual block (380).

Moreover, video decoder 300 may filter the decoded block (382), e.g.,using cross-component adaptive loop filtering according to any of thetechniques of this disclosure. Video decoder 300 may then store thefiltered block (384), e.g., for reference when predicting a future blockto be decoded.

FIG. 8 is a flowchart illustrating an example method for cross-componentadaptive loop filtering (CC-ALF) on a current block in accordance withone or more techniques of this disclosure. The current block maycomprise a current CU. Although described with respect to video decoder300 (FIGS. 1 and 4), it should be understood that other devices may beconfigured to perform a method similar to that of FIG. 8.

Video decoder 300 may decode a plurality of filter coefficients of across-component adaptive loop filter (802). For instance, to decode aparticular filter coefficient of the plurality of filter coefficients,entropy decoding unit 302 may decode, from an encoded video bitstream, asyntax element specifying an exponent value that represents a log base 2of an absolute value of the particular filter coefficient as two raisedto the power of the exponent value. Where the exponent value is non-zero(i.e., has a value other than zero), entropy decoding unit 302 maydecode from the encoded video bitstream, a syntax element specifying asign of the particular filter coefficient. Entropy decoding unit 302 maydetermine a value of the particular filter coefficient based on theexponent value (and the sign value where present). For instance, entropydecoding unit 302 may determine the value of the particular filtercoefficient in accordance with the following equation:c(i)=sign(i)*2^(c′(i)+1)

where c(i) is the value of the particular filter coefficient, sign(i) isnegative one where the sign is negative and positive one where the signis positive, and c′(i) is the exponent value for the particular filtercoefficient.

Video decoder 300 may reconstruct samples of a block of video data(804). For instance, video decoder 300 may reconstruct the samples asdescribed above with reference to FIG. 7. As one example, video decoder300 may add samples of a predictor block with residual data toreconstruct the samples of the block.

Video decoder 300 may perform cross-component adaptive loop filtering,based on the plurality of filter coefficients, on the block of videodata (806). For instance, as discussed above, a CC ALF Cb filter and aCC ALF Cr filter of filter unit 312 (e.g., CC ALF Cb filter 510 and CCALF Cr filter 512) may generate enhancement chroma blocks bybit-shifting, based on values of the plurality of filter coefficients,samples of the block of video data without performing multiplication. Inthis way, the techniques of this disclosure may reduce system resourcesrequired to perform CC-ALF.

The following numbered examples may illustrate one or more aspects ofthe disclosure:

Example 1

A method of decoding video data, the method comprising: coding aplurality of filter coefficients of a cross-component adaptive loopfilter, wherein values of one or more of the plurality of filtercoefficients are constrained to be zero or a power of two; bit-shiftingthe values of one or more of the plurality of filter coefficients areconstrained to be zero or a power of two; coding a block of video data;and performing cross-component adaptive loop filtering of the decodedblock using the filter coefficients.

Example 2

The method of example 1, wherein values of all of the plurality offilter coefficients are constrained to be zero or a power of two.

Example 3

The method of example 1, wherein at least one value of the plurality offilter coefficients is not constrained to be zero or a power of two.

Example 4

The method of any of examples 1-3, wherein performing thecross-component adaptive loop filtering comprises not multiplying filtercoefficients with values of zero or powers of two by samples of thedecoded block.

Example 5

The method of any of examples 1-4, further comprising: coding one ormore syntax elements that indicate values of filter coefficients of theplurality of filter coefficients that are constrained.

Example 6

A method of decoding video data, the method comprising: determining anumber of bits, k, used to represent a decimal value for a filtercoefficient of a cross-component adaptive loop filter; determining thata dynamic range of the filter coefficient comprises (−(1<<(k−j)),(1<<(k−j))−1); coding a block of video data; and performingcross-component adaptive loop filtering of the decoded block using thefilter coefficient.

Example 7

The method of any of examples 1-6, wherein coding comprises decoding.

Example 8

The method of any of examples 1-7, wherein coding comprises encoding.

Example 9

A device for coding video data, the device comprising one or more meansfor performing the method of any of examples 1-8.

Example 10

The device of example 9, wherein the one or more means comprise one ormore processors implemented in circuitry.

Example 11

The device of any of examples 9 and 10, further comprising a memory tostore the video data.

Example 12

The device of any of examples 9-11, further comprising a displayconfigured to display decoded video data.

Example 13

The device of any of examples 9-12, wherein the device comprises one ormore of a camera, a computer, a mobile device, a broadcast receiverdevice, or a set-top box.

Example 14

The device of any of examples 9-13, wherein the device comprises a videodecoder.

Example 15

The device of any of examples 9-14, wherein the device comprises a videoencoder.

Example 16

A computer-readable storage medium having stored thereon instructionsthat, when executed, cause one or more processors to perform the methodof any of examples 1-8.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processingcircuitry,” as used herein may refer to any of the foregoing structuresor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of decoding video data, the methodcomprising: decoding a plurality of filter coefficients of across-component adaptive loop filter, wherein decoding a particularfilter coefficient of the plurality of filter coefficients comprises:decoding, from an encoded video bitstream and using a fixed length code,a syntax element specifying an exponent value that represents a log base2 of an absolute value of the particular filter coefficient as tworaised to the power of the exponent value; and determining a value ofthe particular filter coefficient based on the exponent value;reconstructing samples of a block of video data; and cross-componentadaptive loop filtering, based on the plurality of filter coefficients,the block of video data.
 2. The method of claim 1, wherein absolutevalues of all of the plurality of filter coefficients are constrained tobe zero or a power of two.
 3. The method of claim 1, wherein decodingthe particular filter coefficient further comprises: responsive to theexponent value being a value other than zero, decoding, from the encodedvideo bitstream, a syntax element specifying a sign of the particularfilter coefficient, wherein determining the value of the particularfilter coefficient further comprises determining the value of theparticular filter coefficient based on the sign.
 4. The method of claim3, wherein determining the value of the particular filter coefficientcomprises determining the value of the particular filter coefficient inaccordance with the following equation:c(i)=sign(i)*2^(c′(i)+1) where c(i) is the value of the particularfilter coefficient, sign(i) is negative one where the sign is negativeand positive one where the sign is positive, and c′(i) is the exponentvalue for the particular filter coefficient.
 5. The method of claim 1,wherein the cross-component adaptive loop filtering comprisesbit-shifting, based on values of the plurality of filter coefficients,samples of the block of video data without performing multiplication. 6.A method of encoding video data, the method comprising: encoding valuesof a plurality of filter coefficients of a cross-component adaptive loopfilter, wherein encoding a value of a particular filter coefficient ofthe plurality of filter coefficients comprises: encoding, in an encodedvideo bitstream and using a fixed length code, a syntax elementspecifying an exponent value that represents a log base 2 of an absolutevalue of the particular filter coefficient as two raised to the power ofthe exponent value; reconstructing samples of a block of video data; andcross-component adaptive loop filtering, based on the values of theplurality of filter coefficients, the block of video data.
 7. The methodof claim 6, wherein absolute values of all of the plurality of filtercoefficients are constrained to be zero or a power of two.
 8. The methodof claim 6, wherein encoding the particular filter coefficient furthercomprises: responsive to the particular filter coefficient having avalue other than zero, encoding, in the encoded video bitstream, asyntax element specifying a sign of the particular filter coefficient.9. The method of claim 6, wherein cross-component adaptive loopfiltering comprises bit-shifting, based on values of the plurality offilter coefficients, samples of the block of video data withoutperforming multiplication.
 10. A device for decoding video data, thedevice comprising a memory configured to store at least a portion of anencoded video bitstream; and one or more processors that are implementedin circuitry and configured to: decode a plurality of filtercoefficients of a cross-component adaptive loop filter, wherein, todecode a particular filter coefficient of the plurality of filtercoefficients, the one or more processors are configured to: decode, fromthe encoded video bitstream and using a fixed length code, a syntaxelement specifying an exponent value that represents a log base 2 of anabsolute value of the particular filter coefficient as two raised to thepower of the exponent value; and determine a value of the particularfilter coefficient based on the exponent value; reconstruct samples of ablock of video data; and cross-component adaptive loop filter, based onthe plurality of filter coefficients, the block of video data.
 11. Thedevice of claim 10, wherein absolute values of all of the plurality offilter coefficients are constrained to be zero or a power of two. 12.The device of claim 10, wherein, to decode the particular filtercoefficient, the one or more processors are further configured to:responsive to the exponent value being a value other than zero, decode,from the encoded video bitstream, a syntax element specifying a sign ofthe particular filter coefficient, wherein, to determine the value ofthe particular filter coefficient, the one or more processors arefurther configured to determine the value of the particular filtercoefficient based on the sign.
 13. The device of claim 12, wherein, todetermine the value of the particular filter coefficient, the one ormore processors are configured to determine the value of the particularfilter coefficient in accordance with the following equation:c(i)=sign(i)*2^(c′(i)+1) where c(i) is the value of the particularfilter coefficient, sign(i) is negative one where the sign is negativeand positive one where the sign is positive, and c′(i) is the exponentvalue for the particular filter coefficient.
 14. The device of claim 10,wherein, to cross-component adaptive loop filter, the one or moreprocessors are configured to bit-shift, based on values of the pluralityof filter coefficients, samples of the block of video data withoutperforming multiplication.
 15. A device for encoding video data, thedevice comprising a memory configured to store at least a portion of anencoded video bitstream; and one or more processors that are implementedin circuitry and configured to: encode values of a plurality of filtercoefficients of a cross-component adaptive loop filter, wherein, toencode a value of a particular filter coefficient of the plurality offilter coefficients, the one or more processors are configured to:encode, in the encoded video bitstream and using a fixed length code, asyntax element specifying an exponent value that represents a log base 2of an absolute value of the particular filter coefficient as two raisedto the power of the exponent value; reconstruct samples of a block ofvideo data; and cross-component adaptive loop filter, based on thevalues of the plurality of filter coefficients, the block of video data.16. The device of claim 15, wherein absolute values of all of theplurality of filter coefficients are constrained to be zero or a powerof two.
 17. The device of claim 15, wherein, to encode the particularfilter coefficient further, the one or more processors are furtherconfigured to: responsive to the particular filter coefficient having avalue other than zero, encode, from the encoded video bitstream, asyntax element specifying a sign of the particular filter coefficient.18. The device of claim 15, wherein, to cross-component adaptive loopfilter, the one or more processors are configured to bit-shift, based onvalues of the plurality of filter coefficients, samples of the block ofvideo data without performing multiplication.